Cucurbit residue infested with
the gummy stem blight fungus (known as Dymella Bryoniae) has been
known to be an important source of the pathogen for more than 80 years.
A 1922 USDA Farmer’s Bulletin encouraged growers to rotate their
watermelon fields to reduce carry over (or survival) of this fungus.
Crown cankers are more common on cantaloupe than on watermelon. Wet
greasy , tan portions of cankers can continue to expand and weaken
plants. Dry, corky cankers no longer expand but can have fungus present
as tiny, black fruiting bodies that produce spores during wet periods.
In a study conducted during 1996 - 1999, the gummy stem blight fungus
was found to survive up to 32 weeks (or longer than seven months)
in buried watermelon vines. At the end of that study, however, the
question remains, would the fungus survive longer if it were present
in cankers on the thick crowns of plants.
Another study was initiated in June 2002 . Several hundred cankered
crowns from a cantaloupe field in South Carolina were either 5 inches
deep or left on the soil surface. After 35 weeks, none of the buried
crowns had any gummy stem blight fungus left alive. This result was
similar to a previous study. However, it was alarming to find at 48
weeks - 11 months after the crowns were place in the field - more
than 60 percent of the crowns left on the surface still had living,
active pathogen in or on them.
This was repeated in December 2003 to October 2004. Again, a few of
the cankers - this time only 6 percent - that were left on the soil
surface still had living gummy stem blight in them. In this study,
the fungus survived longer than 30 weeks but less than 45 weeks in
buried crowns. There was no change in survival during the first 15
weeks in this study, because this study was started in the winter
instead of the summer.
In both years, the gummy stem blight fungus survived longer in buried
crown cankers, 9.2 months, than in buried vines, 7.4 months. In addition
the fungus survived longer than 10.7 months on the soil surface. In
both experiments, the scientist ran out of crowns to sample before
all of the fungus was dead. In the 1996 - 1999 study with vines, the
fungus died our before the vines decayed completely. With crowns,
this was not the case. Some of the fungus was present as long as crown
tissue could be found.
In 2004 - 2005, an additional treatment was added: placing crowns
on the top of plastic-covered beds. This placement most closely reproduced
the case in the commercial field. After 17 weeks, 67 percent of the
crowns on the plastic still had living gummy stem blight fungus, but
only 20 percent of the buried crowns did. In another experiment, cantaloupe
plants were inoculated with the gummy stem blight fungus in september
2003. The vines were covered with many fruiting bodies when they died
and were left on top of the raised beds covered with white-on-black
plastic mulch. This vine debris, which was not in direct contact with
soil, was sampled in May, June and July 2004. Again the fungus survived
for nearly 10 months. About 15 percent of the vine pieces still had
living fungus at the last sampling. After July, the debris was so
decayed it was not possible to collect enough to sample again.
Summary. The gummy stem blight fungus
survived best on top of plastic-covered raised beds, not as well on
the soil surface, and worse buried 5 inches deep in soil. The important
point to note is that the fungus can survive for 11 months in the
field. Under the normal cropping cycle in South Carolina, some of
the gummy stem blight fungus that is present in crowns of watermelon
or cantaloupe at the end of the season in June or July will still
be there if the same field is replanted in March or April the following
year. And clearly, if a spring watermelon field is cropped to pumpkin
or cucumber in the fall, the fungus will be there! One of the most
cost-effective ways to control gummy stem blight is to disk cucurbit
fields immediately after harvest to reduce survival of the pathogen.
Two years out of cucurbits, counted from planting date to planting
date, should be considered a minimum for any field. Because some gummy
stem blight fungus was still alive at the end of the 2002-2003 and
2003-2004 experiments, a short rotation time between cucurbit crops
cannot be recommended at this time.
